Digging into it it seems to be related to the problem described in the
commit message for cd998ded5c12 "i2c: designware: Prevent runtime
suspend during adapter registration" as scsi_add_host also calls
device_add and after the call to device_add the parent device is
suspended.

Fix this by using the approach from the mentioned commit and getting
the runtime pm reference before calling scsi_add_host.